Although the BMW 6-Series designation had been last applied to a BMW in 1989, BMW has a long tradition of sport coupes and convertibles of the highest caliber. In the spirit of this tradition, BMW introduced a new 6-Series in 2004, consisting of two models of surpassing design, technology and excitement.
The 6-Series models are the BMW 645Ci Coupe, at $70,595 including destination charge, and the BMW 645Ci Convertible at $77,595. Each is powered by the brilliant 4.4-liter, 325-hp Valvetronic V8 that had already been winning hearts and minds in the 745i/Li and 545i models.
And since "6" falls between "5" and "7," it will not surprise that this new sport Series is related to the 5 and 7-Series. Yet the 6-Series is anything but a mere re-bodying of either of these sedans. For a perspective on how the new models fit into the upper end of the BMW line, consider the following:
- Compared to the 5-Series, the 6 rides on a wheelbase 4.3 in. shorter, yet is almost as long thanks to the 6's long-hood, cabin-back proportions. Compared to the 745i, the 6-Series wheelbase is 8.3 in. shorter, reflecting the contrast between the 7's large, commodious sedan format and the 6's more intimate, sporting coupe/convertible character.
- A notably wide rear track and low height visually express the 6-Series' sporty handling attributes.
- No sheet metal is shared with either the 5 or 7-Series.
- BMW's most extensive use of weight-saving body materials in regular-production models results in impressive weight efficiency.
- Though all three Series use the Valvetronic V8 engine, the BMW 645Ci Coupe achieves the best acceleration
- Like the 545i but not the 7-Series, both 645Ci models are available with all three BMW transmission types: manual, STEPTRONIC automatic and Sequential Manual Gearbox (SMG). All are 6-speeds.
- Whereas the 5 and 7-Series Sedans are full 5-seaters, the BMW 645Ci models are 2+2s, with generous front seating space and intimate, yet inviting "occasional" rear seating.
- Thanks in part to standard run-flat tires, the BMW 645Ci Coupe provides almost as much trunk space as the 545i; the Convertible trunk, with its variable softtop storage compartment, is also relatively spacious for a car with a folding top.
In 1956, BMW unveiled the 503 coupe and cabriolet, designed by Count Albrecht Goertz - who, in the same period, designed the immortal BMW 507 roadster. Their V8 engine delivered 140 hp from 3.2 liters and powered the 503s to the handsome top speed of 118 mph; in those days as now, one could drive legally at top speed on Germany's Autobahnen. BMW developed one more sport coupe on the V8 chassis; the great Italian designer Bertone styled the 3200 CS, which went into production in '62.
The next coupe, a smaller one, appeared in 1965; its exterior design, especially that of its "greenhouse" or passenger cabin, related closely to Bertone's 3200 CS look. Powered by a 4-cylinder engine in 2-liter form, it came in two forms - 2000 C with 100 hp, 2000 CS with 120 hp.
In 1968, BMW re-entered the 6-cylinder luxury segment with the new 2500/2800 sedans, and a coupe version followed soon thereafter. It was called 2800 CS, and over the years of its production (until '75), it evolved into many forms, including the 3.0 CS, considered by many to be one of the most beautiful BMWs ever made. This Series is often dubbed simply "the CS."
The first 6-Series succeeded the CS just described. Longer, wider and lower than its predecessor, it made its U.S. debut in '77. The 6-Series eventually evolved into the 635CSi and, with the mighty BMW M 6-cylinder engine, the M6.
Though not exactly a lineal ancestor of the new 6-Series, the 8-Series is BMW's most recent performance-luxury coupe. Over the years of its production (1990-97), it appeared in several forms, from the V8 840Ci through the 372-hp, V12 850CSi.
Each of these 6-Series forebears was a unique automobile, distinguished in both engineering and esthetics from its sedan stablemates. The new 6-Series is utterly consistent with that tradition.
The advanced Valvetronic V8 engine
In the new 6-Series, this engine's inherent brilliance is emphasized by a unique exhaust system that endows it with an especially sporty sound. Here, as in other BMW automobile models where it's offered, the 4.4-liter V8 delivers 325 hp and 330 lb-ft. of torque - all the more impressive in the context of vehicle weights as low as 3781 lb. (for the BMW 645Ci Coupe with manual transmission). Indeed, from a standstill the Coupe can reach 60 mph in just 5.5 seconds with the manual transmission or SMG.
Valvetronic: revolutionary "breathing" concept. BMW engines achieve variable valve timing by rotating the camshafts relative to their driving sprockets. Varying valve lift is a step beyond varying timing. Valvetronic varies lift - but to a far greater, and more fundamental, degree than any other system. Indeed, engine breathing is controlled entirely by the valves; the conventional throttle simply goes away.
The Valvetronic mechanism sits atop the intake valves on each of the V8's two cylinder banks. Each of the 32 valves is actuated as the camshaft lobe deflects a finger-type rocker arm. On the intake side, there is an additional element between the cam lobe and rocker arm, an intermediate follower.
Upon contact by the camshaft lobe, this intermediate follower actuates the rocker arm and, in turn, the valve. The follower is held in place by an eccentric shaft, rotated by a small servo motor in response to the driver's accelerator-pedal movements. This determines the intermediate follower's pivot point and thus varies the valve lift.
Double VANOS rotates the intake and exhaust camshafts steplessly between "earliest" and "latest" valve timing to enhance the engine's torque, fuel efficiency and emission control. Here it operates in combination with Valvetronic to help achieve outstanding levels of performance, efficiency and general operational excellence.
Fully variable intake manifold. Many engines, including BMW's own 6-cylinder units, employ 2-stage intake manifolds. Generally, these switch between two paths for air entering the engine: one tuned for low- to medium-speed operation (improving torque and response), the other improving top-end power.
For the N62 engine, BMW engineers evolved this concept into a fully, steplessly variable intake manifold. Inside the manifold is an internal mechanism consisting of two intertwined helical elements which, rotated by an electric servo motor, vary the effective intake length steplessly. Like Valvetronic and stepless Double VANOS, this manifold concept dispenses with traditional compromises to achieve truly optimum performance.
The exhaust system is specially engineered to lend the V8 engine an extra-sporty note. Actuated by engine vacuum in response to engine speed and load as well as the gear currently engaged, a movable element in the right-hand resonator is programmed to achieve the sportiest, most pleasurable engine sound within the overall exterior-noise limit of 74 decibels (dBA).
Three available transmissions, all 6-speeds
6-speed manual. The standard transmission is the heavier-duty version of two new 6-speeds introduced in the past year. Compared to the 6-speed used in M3models, it is lighter, yet robust enough to handle the greater torque of the V8 engine.
6-speed STEPTRONIC automatic. This combination of torque converter and 6-speed gearbox is plenty sporty - as demonstrated by a 0-60-mph time only slightly less quick than for the manual transmission or SMG. As with all other current BMW automatics, the S (Sport) mode moves shift points upward (to higher engine speeds) for sportier feel and response. The M (Manual) mode enables the driver to up- or downshift at will, imparting some of the sportiness of a manual transmission. Driver-controlled up- and downshifts are effected by "tipping" the shift lever rearward or forward.
Sequential Manual Gearbox (SMG): the "third way" of driving. This is an electrohydraulically shifted, electronically controlled version of the 6-speed manual transmission, including an automatic clutch; as such it is utterly different from a conventional automatic. There is no clutch pedal; the driver selects the desired mode (N, R, D, S) with a console-mounted selector lever, and can execute manual shifts via that lever or two "paddles" on the steering wheel. SMG's fundamental advantages are that it fully preserves the performance of a manual transmission while facilitating both automated and very sporty driving.
Features and characteristics of the SMG driver interface includes:
- A Drive mode (D) in which shifting is automated
- A Sequential mode (S) in which shifting is mostly driver-controlled
- A Sport program, selected by a Sport button on the console that also affects other performance and handling characteristics.
- An instrument-panel display of the gear currently engaged and the operational range currently selected.
In D, shifts are automatic and adaptive: More aggressive driving results in shift points at higher speeds. D is not to be interpreted as a substitute for the D of a fully automatic transmission, but rather a convenient operational mode for those times when the driver favors ease of driving over extracting maximum performance.
With S selected, the driver essentially controls all shifting with the shift lever or paddles:
- Tip lever or paddle(s) rearward = upshift
- Tip lever or paddle(s) forward = downshift.
An instrument-cluster display tells the driver which of the six speeds is currently engaged. In D, it also shows a "D."
Dynamic Driving Control
BMW's Dynamic Driving Control is standard. When the driver selects its Sport program via the Sport button on the center console, an LED in the button illuminates and vehicle behavior is modified as follows:
- The engine's response to the accelerator is "quicker"; a given amount of accelerator movement produces more response. (Cruise-control operation is similarly quicker.)
- With SMG in Sequential, manual shifts occur more quickly (sharply).
- With SMG in Drive, automated shifts occur more quickly and at higher road speeds.
- With the STEPTRONIC automatic transmission in Drive, automatic shifts occur at higher road/engine speeds.
- With the STEPTRONIC automatic transmission in its normal Sport mode (as engaged with the shift lever), automatic shifts occur at even higher road/engine speeds; this is thus a sort of "super-sport" mode.
- Steering power assist (with standard Servotronic steering only) is reduced.
Aluminum driveshaft
By using aluminum for the 2-piece driveshaft, BMW engineers achieved a significant (6.6-lb.) weight reduction. At the same time, they were able to improve the vehicle's management of impact energy in a frontal crash: both shaft sections are designed to collapse in a controlled manner under impact load. Connecting elements at the ends of each shaft section are of steel; new welding techniques were developed to weld the aluminum to the steel.
Front suspension: full aluminum, specific 6-Series kinematics
The front suspension system is a further evolution of BMW's aluminum double-pivot front system. Its general concept and design are shared with the 5 and 7-Series, but specific characteristics, dimensions and kinematics - including the 6's lower ride height and center of gravity - endow the 645Ci models with their own, definitively sporty handling and riding characteristics. Most of its components are of aluminum, which reduces unsprung weight and thus optimizes the suspension's response to irregular road surfaces.
Key distinctions between the 6 and 5-Series front suspension (BMW 645Ci Coupe/Convertible vs. 545i) include:
- Stronger self-centering feel in steering
- Lower ride height
- Firmer springs and shock absorbers compared to standard 545i calibration, not quite as firm as 545i sport calibration; the 6-Series offers just one calibration, and it is sporty.
Rear suspension system: here too, specific calibration of a proven system
Like the 5 and 7-Series, the 6 employs BMW's most premium rear suspension system, a multi-link layout called Integral Link. This system controls rear-wheel angles very precisely, minimizing unwanted effects under load changes (such as lifting off the gas while cornering, and hard acceleration or braking) - and achieves a remarkably comfortable ride.
Here too, aluminum is extensively employed. Because supple reaction of the suspension to bumps is especially critical for good road adhesion at the powered rear wheels, the benefits are if anything more important here than at the front.
Key distinctions between the 6 and 5-Series rear suspension (BMW 645Ci Coupe/Convertible vs. 545i) include:
- Wider rear track, 62.7 vs. 62.3 in.
- Greater negative camber, 2 vs. 1.5 degrees
- Lower roll center
- Lower ride height.
Standard Active Roll Stabilization
This high-tech suspension innovation dramatically reduces body roll in cornering. In so doing, ARS improves handling by virtue of better suspension geometry (wheel angles relative to vertical), but there is also a psychological component: Drivers and passengers alike marvel at the resulting "flat cornering." The ARS system consists of:
- Active anti-roll bars, replacing conventional mechanical ("passive") front and rear bars. Each bar consists of left and right portions, twisted in opposite directions by a hydraulic motor between them.
- A valve/sensor block containing various system valves and sensors.
- A lateral-acceleration sensor to detect how hard the vehicle is cornering.
- An electronic control unit (ECU) regulating the entire system.
- A tandem oil pump which, via its two sections, provides hydraulic pressure for ARS and the power steering.
- An oil cooler, reservoir, filter, oil-level sensor and the various hoses, mounting brackets and other minor components.
Whenever the vehicle enters a corner, curve or avoidance maneuver, lateral acceleration is generated. This is read by the sensor, which transmits a signal to the ECU. The ECU processes this signal and transmits it to the valve/sensor block. In turn, the valve/sensor block determines the hydraulic pressure applied to the active anti-roll bars to control body roll.
Active Roll Stabilization
- Generates resistance to body roll by twisting the anti-roll bars.
- Does so in a stronger and more highly "tailored" way than conventional anti-roll bars.
- Offers no resistance to bumps in straight-ahead driving, as do conventional anti-roll bars; this improves riding comfort.
- Increases the vehicle's maximum cornering capability.
- Improves steering response, particularly in the range of cornering where body roll is most tightly controlled.
This comment from Car and Driver's January '04 drive of the BMW 645Ci Coupe nicely captures the effectiveness of the 6-Series' chassis design: "Flung at reckless speeds up the winding roads of Andalusia, the 645Ci is serenely stable and solid. It pounces on corners, tracks flat through the apexes, and devours the straights with a burly snarl from the V8. It laps up freeway kilometers at triple-digit speeds, the suspension digesting ripples and dips so thoroughly that the body remains almost inert. All the driver has to do is aim the 645 and it goes there. Fast."
Rack-and-pinion steering with Servotronic power assist and variable ratio
Like the 7-Series, the 6 has a variable-ratio rack-and-pinion system with Servotronic vehicle-speed-sensitive variable power assist. (In the 5-Series, Servotronic comes with Active Steering, as part of the Sport Package.)
The variable-ratio feature (not to be confused with variable assist, nor with Active Steering's more dramatically variable ratio) is achieved by special profiling of the rack-and-pinion mechanism's gear teeth. This causes the steering ratio to become quicker (that is, more steering effect for a given steering-wheel motion) as the wheel is turned outward from its center position. The result is a fine-tuning of steering response according to the situation one is in, be it on a fast straightaway (slower steering) or maneuvering into a parking space (quicker steering).
Active Steering: dramatic advance in vehicle dynamics
Active Steering is included in the Sport Package for each model, and is also newly available as a stand-alone option for '05. It expands dramatically on the variable-ratio principle, and is described under options & accessories.
Innovative brakes: abundant stopping power, new weight-saving technology
An innovative, weight-saving construction of the brake discs is standard on both 6-Series models. In this construction, patented by BMW, the brake disc consists of a high-carbon cast-iron outer portion, which functions conventionally as the surface onto which the brake pads grip; and an aluminum center "hat," which mounts the rotor to the vehicle. The concept's advantages include:
- Reduced unsprung weight, complementing the aluminum suspension. Compared to conventional all-cast-iron rotors, actual weight reductions are a very significant 2.2 lb. at the front, 1.5 lb. at the rear.
- Reduced rotor deformation under hard braking, by approximately 20%. This means less tendency of the brakes to vibrate when hot, and reduced likelihood of the rotors cracking under extreme heat conditions.
The front brake calipers are of aluminum, which further reduce unsprung weight. Thus with the 6-Series, BMW is once again taking the lead in combining top handling with amazing riding comfort.
18- or 19-in. wheels and tires, run-flat standard
Standard 6-Series wheel/tire equipment encompasses 18 x 8.0 cast-alloy wheels in a handsome, discreetly sporty Radial Spoke design; these carry 245/45R-18 V-rated run-flat all-season tires that give standard-equipped 645Ci models excellent all-around performance and traction.
Included in the Sport Package are 19-in. wheels in an even sportier Ellipsoid design; these are 8.5 in. wide at the front, 9.0 wide at the rear. At 245/40R-19, the front tires are the same width as the standard ones; the rears move up to 275/35R-19. Also of run-flat construction, these Sport Package tires are W-rated performance-type, sometimes referred to as "summer" tires.
Although run-flat tires have acquired a reputation for a relatively hard ride, BMW's talented handling-and-ride engineers have, typically, risen above the usual compromises. "Sporty and comfortable," concluded Germany's Auto Zeitung in its October 22, '03 issue; "The firm but not uncomfortable chassis supports a dynamic style of driving." Giving ARS some of the credit but coming to the same conclusion, the U.S. AutoWeek reported in its November 24, '03 issue: "ARS allows for the springs and dampers to be set for a comfortable ride, with the system's hydraulically operated anti-roll bars front and rear stabilizing things when needed."
A Flat Tire Monitor is standard to alert the driver to any significant change in a tire's air pressure. Because all its models come with run-flat tires, the 6-Series was designed without a spare wheel/tire; this contributes to the Series' generous trunk capacity.
Dynamic Stability Control in its latest form
DSC appears in its latest form (version 8.0) in the 6-Series, thus incorporating various recent programming refinements and the Dynamic Traction Control function. Activated by pressing the DSC console switch briefly, DTC improves utilization of available road traction under specific conditions
- on sand, gravel, deep snow or packed snow
- climbing hills with deep or packed snow
- when there is deep snow on only one side of the road
- when driving with tire chains.
With DTC selected, engine intervention is de-activated at low speeds, leaving only the individual wheel brakes to control wheelspin. When the vehicle reaches a speed of approximately 43 mph, normal DSC operation is re-instated until speed once again drops below this threshold, at which point DTC operation resumes. A longer push of the DSC button de-activates all DSC functions except antilock braking.
The look: esthetics and function
With this performance-luxury Coupe and Convertible pair, the Bavarian automaker shows in yet another way "what it can do" in terms of contemporary design. From the bold BMW "kidney" grilles and the freely shaped lighting clusters flanking them...through a very long hood signifying power...through a sleek and graceful silhouette...to BMW's characteristic (and "highly" functional) rear deck: This is BMW in the 21st century.
The primary aerodynamic goal has been achieved here, as demonstrated by an excellent coefficient of aerodynamic drag (CD) of 0.30 (Coupe) or 0.32 (Convertible) and a highway EPA mileage rating of 26 mpg with automatic transmission for both models.
Another goal, low aerodynamic lift, was achieved in a fascinating way. The main upper trunklid line begins in the C-pillar and defines its rear edge all the way around. Above this, across the center of the lid, there's a rise - a spoiler lip, just a few millimeters tall. Wind-tunnel tests indicated the necessity for this; Boyer's designers drew an ideal shape for it. But it couldn't be formed in steel or aluminum. The solution: make the trunklid out of Sheet Molding Compound, or SMC.
Other functional elements include BMW's now across-the-board Adaptive brakelights, which give off more illumination under hard braking or anytime the ABS intervenes. And in the front flanks, BMW incorporates additional turn-signal indicators, so-called "repeat blinkers." Here these are especially elegant, set flush into a recess that forms the origin of a side character line. The designers wanted this refined solution; like the rear spoiler lip, it couldn't be stamped into steel or aluminum. The solution: make the front side panels, or "fenders" in older parlance, of thermoplastic.
Technology of the body: strong and light structure
Though the front-end structure and hood are of aluminum as on the 5-Series, the front side panels are of thermoplastic, which saves weight similarly to aluminum. The doors are aluminum, vs. steel on the 5-Series; they save yet more poundage. Finally, the SMC rear deck, also a weight-saver, offers extra resistance to bending - important in a large-area component like this. (Structural Engineer Ralf Grünn notes also that in a 50-mph rear-impact crash test, the trunklid did not break.)
The hood's aluminum outer and inner skins - the latter essential for strength - are bonded together with a new adhesive that gives it both longitudinal strength (a factor in crash-energy absorption) and vibration resistance. Formerly, two different adhesives were necessary for these two functions; using only one saves 6.2 lb.
In its lightest form, with manual transmission, the BMW 645Ci Coupe weighs 3781 lb., 22 lb. less than the 545i Sedan similarly equipped. In fact, the Coupe body/chassis unit, complete with doors, hood and trunklid, weighs just 728 lb., vs. 762 lb. for the all-steel 330Ci Coupe. Weight distribution for the Coupe is 51.6% front/48.4% rear - excellent and in keeping with BMW's policy of relatively even front/rear distribution.
Aerodynamics: not just on the surface, but also underneath
The 6-Series' exterior shape certainly implies excellent aerodynamics. Yet there are also details that help foster smooth progress through the air.
One such detail is the extensive underbody fairing, reaching from just behind the front bumper nearly back to the rear suspension. The material used for most of the underbody fairing is Superlite, which is some 40% lighter than materials previously used for such applications.
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